The present invention relates to the method of manufacturing a lead frame, particularly that can be manufactured using a clad sheet formed by cold press-bonding method.
Recently, accompanied with semiconductor device being more highly accumulated, having more pins and being smaller sized, and electronic equipment being smaller sized and lighter weighted, high density mount board is required.
In the method for connecting semiconductor chips to lead frames, a metal lead having bumps, which could minimize the assembly process, the size of package and the cost compared to the conventional wire-bonding method, has been researched and developed. In this method, jutting bumps are formed at the edge portion of a lead and the lead connects to semiconductor chip intervening theses bumps, which can make a package thin.
And now, as a method for forming bumps, a three layered clad sheet is tried to be used as a metallic material for manufacturing lead frame.
Japanese laid open publication No. Hei 9-115965 discloses a method of manufacturing a lead frame by using such three layer clad sheet. In this manufacturing method, a clad sheet is manufactured by holding a high hardness copper sheet between pure copper sheets.
However, in the method mentioned above, any etching stopper layer does not exist in the clad sheet, which causes a problem that the control of the chemical etching is extremely difficult.
There is also a method of manufacturing lead frame using three layer clad sheets in which aluminum or ferrous alloy is used as an etching stopper layer. But these clad sheets are formed by a usual cold rolling method using a higher reduction rate, which produce unflat bonding surface or form an alloy layer on a surface between a copper sheet and etching stopper layer during heat treatment, which causes inferior selective etching.
In addition, there is the other method of manufacturing a lead frame, in which thin metal film is formed by an vapor deposition method. However, there was a problem, that is, in case that the formed film is thin (several xcexcm), pores are easily generated, while in case that the formed film is thick (10 xcexcm or more), the productability is deteriorated and the cost becomes high.
The present invention is aimed to solve such problems, and the objective of the present invention is to produce a clad sheet for lead frame which can be manufactured at low cost and has an excellent selective etchability, a lead frame using thereof, and a manufacturing method thereof.
The clad sheet for lead frame is characterized in that it is manufactured by press bonding a copper foil to nickel foil at the reduction rate of 0.1 to 3%.
The copper foil has a nickel plating on one or both sides.
The clad sheet for lead frame is characterized in that it is manufactured by press bonding an aluminum foil to a nickel foil at the reduction rate of 0.1 to 3%.
The clad sheet for lead frame is characterized in that it is manufactured by press bonding a copper foil having nickel plating on one side of it to an aluminum foil at the reduction rate of 0.1 to 3%.
According to still another feature, the clad sheet for a lead frame is characterized in that it is composed of a three-layer construction comprising copper/nickel/copper.
According to a further feature, the clad sheet for a lead frame is alternatively characterized in that it is composed of a three-layer construction comprising copper/nickel/aluminum.
According to a still further feature, the lead frame is characterized in that bumps are formed by selectively etching the clad sheet.
The method of manufacturing the lead frame is preferably characterized by forming a clad sheet for lead frame by press-binding a copper foil or an aluminum foil to a nickel foil or a nickel copper foil which serve as an etching stopper layer, and then forming bumps of copper or aluminum by selectively etching the clad sheet.
The manufacturing method for the lead frame is preferably characterized in that:
said clad sheet for lead frame is formed by
previously practicing an activation treatment of the bonding surface of said copper foil or aluminum foil and the bonding surface of said nickel foil or nickel plating in a vacuum chamber,
laminating said copper foil or aluminum foil to said nickel foil and nickel plating, and then press-bonding at the reduction rate of 0.1 to 3%, wherein
said activation treatment is practiced
{circle around (1)} in a hyper low pressure inert gas atmosphere of 1xc3x9710xe2x88x921 to 1xc3x9710xe2x88x924 Torr,
{circle around (2)} by glow discharging charging an alternate current of 1 to 50 MHz between one electrode A comprising said copper foil or aluminum foil having bonding surface and said nickel foil or nickel plating respectively electrically ground and the other electrode B insulatingly supported, and
{circle around (3)} by sputter-etching
{circle around (4)} in the manner that the electrode area exposed in plasma generated by said glow discharging is not more than ⅓ the area of electrode B.